Natural gas has significant potential for providing an abundant, low cost alternate fuel for automotive use. One of the problems with natural gas as an alternate fuel for motor vehicles is that the composition of natural gas varies substantially from location to location and with the time of the year. Internal combustion engines tuned to operate efficiently on one composition of natural gas may operate poorly on another.
Natural gas supplies across North America range from 78% to 98% methane. This variation in composition results in problems in optimal performance of an automotive engine using natural gas as a fuel. Knock resistance of a gaseous fuel is measured in terms of the methane number, an analog to the octane number used to rate the anti-knock character of liquid fuels. For the range of natural gas available, the methane number ranges from 73 to 96. An engine that has been calibrated for optimal fuel economy and low tailpipe emissions for a particular composition or methane number of natural gas may function poorly on a different composition of natural gas.
Thus, knowing the methane number of a given fuel can assist in the periodic or continuous adjustment of motor vehicles using natural gas.
Since the molecular weight of methane is low, sound waves travel faster in pure methane than in methane combined with higher molecular weight gases such as nitrogen or ethane that often are present, in natural gas. Thus, the speed of sound in the gas can be used to determine the composition of the natural gas. The speed of sound in the gas also correlates to the methane number of the gas for the range of compositions typical of natural gas.
The present invention is based upon the above principles.